From the Super Mario Wiki
Gnaw, known as Growl in the PAL version, is a large, imposing creature that is purple in color, with blue spots covering its body, several rows of sharp teeth, and some retractable spikes on its head (which will not actually deal any damage to Mario or Princess Peach if they jump on them). Gnaw is also immune to any form of physical harm, making it unable to be defeated.
Gnaw is Merlee's favorite pet, which she also uses to guard her mansion. In Super Paper Mario, when Mimi took over Merlee's Mansion she chained the Gnaw to a door, making it incapable of attacking her. If Mario or Princess Peach were to approach Gnaw, it would try to grab them in its mouth; if Gnaw succeeded, it would give its catch to some Gnips, who would proceed to throw them out of Merlee's Mansion. The reason it did this was to ensure that Mario and Peach did not get hurt by Mimi, as it knew that Mimi was evil, whereas the heroes were fooled into thinking she was Merlee's maid.
If Mario or Princess Peach managed to obtain a hidden key in Merlee's Mansion, they would need to sneak by Gnaw and use it on a nearby door. Unlocking the door also frees Gnaw, who proceeds to chase a nearby Mimi all throughout Merlee's Mansion (a reference to how the similarly dog-like Chain Chomps chased away Tutankoopa, the Chapter 2 boss of the first Paper Mario). It is not seen again after this.
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